Telephone system



Jan. 23, 1934.

E. C. OLIVER TELEPHONE SYSTEM Filed Dec. 7. 1931 5 Sheets-Sheet l I NPI ML2 ffm?) lbk2 " ZSWHZ ZDL EF1-H2571' Emu-EET. Eli-UE:-

1311.23, 1934. E. c. OLIVER y 1,944,693

TELEPHONE SYSTEM WL. 'ff

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Jal). 23, 1934.

E. c. OLIVER 1,944,693

.TELEPHONE SYSTEM Filed Dec. '7. 1931 5 Sheets-Sheet 3 Hal. 56.2. 56.3. 56.4.

Jan 23, 1934. E. c. OLIVER TELEPHONE SYSTEM Filed Dec. '7. 1931 5 sheets-sheet 4 Jan. 23, 1934. E. C. @Ln/R 1,944,693

TELEPHONE SYSTEM Filed Dec. '7. 1931 5 Sheets-Sheet 5 Erna' En uri-rm 'liuar amf- Patented Jan. 23, 1934 TELEPHONE SYSTEM Ernest Conrtrnan Oliver, Cheshire, England, as-

signor to Associated Telephoneand Telegraph Company, Chicago, Ill., a corporation of Dela- Ware Application December 7, 1931, Serial No. 579,505, and in Great Britain December 12, 1930 16 Claims. (C1. 179-16) The present invention relates to telephone systems and as regards certain of its features is more particularly concerned with small automatic exchanges of the type which do not provide facilities for connection with the public exchange system and are commonly used in offices, factories and the like. The chief object of the invention is to provide an installation of this type which shall employ a minimum of apparatus and hence will require a minimum of space and which shall also be cheap to manufacture and to maintain.

According to one feature of the invention the amount of equipment required is reduced by arranging that the apparatus which responds. to the impulses dialled by the calling party is common to a large number of lines the holding time of this apparatus being reduced, however, by arranging that lit is not taken into use until the calling party actually moves his dial off-normal in the process of dialling. With this object in view it is preferably arranged that the circuit for seizing the impulse responding equipment is completed over shunt springs of the dial as soon as it is moved from its normai position.

Another feature of the invention relates to the use of a special ringing circuit in which ringing current is fed through the winding of a relay which is operated thereby and in consequence of its operation 'maintains closed a point in the ringing circuit. When the calling party removes his receiver, the switchhook springs open the ringing circuit so that the ringing relay is deenergized and therefore opens a further point in the ringing circuit so as to produce a tripping action. Preferably ringing current is supplied over a special lead individual to each substation and this lead may conveniently form part of the circuit for effecting the seizure of the impulse responding equipment. There is no serious objection to this additional lead in installations of this type since the lines are almost without exception very short.

The advantage of simplicity of maintenance and economy of space is also obtained by the use of an arrangement whereby the current for operating purposes is taken from alternating current mains by means of a suitable rectiiier arrangement and the rectified current is smoothed by means of a smoothing circuit employing electrolytic condensers. Since these condensers operate most efficiently if the applied voltage remains substantially constant, it is arranged that the rectifier output circuit normally includes an artificial load which is removed during the selecting operation when a comparatively heavy current drain is produced by the operation of the counting relays.

When the installation is of small size, for in stance lines, the apparatus is preferably assembled into two units, one consisting of the power equipment comprising the transformer, rectifier and chokes and condensers forming the smoothing circuit and the other consisting of the various control relays by means of which 'the proper connections are set up.

The invention will be better understood from the following description of one method of carrying it into effect reference being had to the accompanying drawings comprising Figs. 1 to 5 which should be arranged as shown in the key.

sheet located in Fig, 6 to form a complete circuit of the invention. 1

Referring now to the separate gures, Fig. 1 shows a subscribers instrument circuit and connecting relays by means of which the telephone may be connected to either connecting circuit, Fig. 2 shows `a further subscribers instrument circuit also possessing two connecting relays which serve asimilar function, Fig'. 3 lshowsthe con-V necting circuits arranged to supply speaking battery to the calling and called subscribers, Fig. 4 shows the selector equipment containingl the counting relay chain responsive tothe dialled impulses, wlile Fig. 5 shows a batteryeliminator circuit and ringing feed which is of substantially conventional form. Y i

Referring now to the detailed circuit operation, it will be assumed that the subscriber on telephone Tl, Fig. 2, wishes to make a call to another subscriber on the system when the switch-board apparatus is idle. Upon the removal of thereceiver from the switchhook,r the contacts lVSWI-Il and 2 will be operated, the former opening the circuit to the ringer lRNG and the latter closing a point in the left-hand speaking conductor which is without effect as regards the operation of the switchboard apparatus until therdial lDL is operated preparatory to the sending of the routing digit. When this is done the shunt springs located to the left-hand side of the main impulsing springs of the dial are operated'to complete the following circuit to the line connecting relay associated with this instrument and designated ILP: earth at the resting armature c4, Fig. 4, conductor 11, resting armature 1Zq6, Fig. 2, resting armature 1Zp6, third conductor extending to the telephone instrument T1, operated shunt springs of dial lDL, resting impulse springs, switchhook spring SWH2, left-hand speaking conductor, resting armature llqLleft-hand wind- SOIl ing of the connecting relay ILP, resting armature lbkl, conductor 12, resting armature swS to battery by way of the low resistance YA in parallel with the comparatively high resistance relay TE which is short-circuited under these circumstances. Relay ILP is therefore operated over the circuit described and at its armature Ilpl opens the circuit to a similar connecting relay ILQ associated with the second connecting circuit, at armature Ilp2 prepares a locking circuit to its right-hand locking winding, at armatures 11133 and 1Zp4 connects up the speaking leads from telephone T1 to the first connecting circuit, at armature 1Zp5 completes a circuit for operating relay ISC, Fig. 3, which Will be described later, at armature IZp6 opens its own initial energizing circuit and at armature Ilp'? opens the incoming switch circuit to the connecting relays ILP and ILQ of the calling telephone TI. y

It should be mentioned that the armatures 1Zp5 and Ilp6 are arranged by adjustment to full a make-before-break function so that a locking circuit may be completed to relay ILP before the opening of its initial energizing circuit, The operating circuit previously mentioned for relay ISC may now be traced from: earth at the resting armature Ifml, Fig. 3, resting armature IfrZ, conductor 13, operated armature 1Zp5, third telephone conductor, operated shunt springs of dial, right-hand speaking conductor, operated armature 11114, conductor 14, resting armature IfbZ, Winding of relay ISC, resting armature lsc, conductor 15 to battery in a parallel circuit which is formed by way of the comparatively high resistance relay TE and also by way of the resting armatures tel and b1 to battery via the resistance YA which is sufficiently low to short-circuit relay TE against its operation under these conditions. Relay ISC in operating, at its light armature Iscl completes an operating circuit for relay IBK whose function will be described later, and also extends earth by way of the comparatively low resistance YC to conductor 16, to provide a locking circuit to the right-hand winding of relay ILP. A parallel circuit may also be traced from earth connected to the left-hand winding ofy relay ILP, resting armature IZqI, operated armature 1Zp3, conductor 17, resting armatures IfbI and 1802, conductor 18, resting arf mature sw2, winding of relay SW to battery. It

will be seen therefore, that operating circuits to relays ISC and SW are completed at substantially the same instant and in order to ensure that relay ISC will always operate before relay SW, the latter is equipped with a copper slug so as to render the relay very slightly slow to operate. This arrangement is desirable in order to make quite sure that the locking circuit to the right-hand winding of the connecting relay` ILP is completed before the initial energizing circuit to the left-hand winding can be opened at the armature sw3 when relay SW is operated' and locked to earth by way of the armatures 1b7c11 and 2117611. Relay SW is therefore locked until a further call is made and since the operating battery by way 'of the resistance YA is transferred to the second connecting group then a subsequent call must be made by way of this group although the first group may have been released meanwhile. Such arrangement therefore vensures the even distribution of the calls over the two connecting circuits.

Returning again to the operation of relay IS C, at armature 1802 the initial energizing circuit L0 the distributing relay SW is transferred by Way of conductor 19 and the resting armature c3 to the impulsing relay I of the selector equipment which is immediately operated and at its armature i1 operates relay B, at armature 1303 an operating circuit is completed to the marking relay IFM, at armature 1804 an alternative locking circuit is completed to relay ISC, at armature 1805 the winding of relay ISC is connected direct to resistance battery by way of YA, and at armature 1506 a point is opened in the initial battery circuit which included relay TE. It should be explained that armatures 1305 and Isc6 are arranged by adjustment to full a make-beforebreak function so as not to interrupt the locking circuit to relay ISC during the changeover of the battery circuit.

' Considering now the operation of relay IBK, it should be mentioned that this relay is provided with a plurality of armatures arranged to open the initial energizing circuits to all the connecting relays LP associated with all the telephone .circuits such as T1, while at a further armature Iblcll an alternative locking circuit is provided to the distributing relay SW. Relay B in operating, at its armature b1 opens the short-circuiting path to relay TE, at its armature b2 prepares a point in the impulsing circuit to the counting relays P to U, at armature b3 completes an operating circuit for relay E on its lower winding and prepares a locking circuit for the counting relays when operated, and at armature b4 completes a locking circuit to relay ISC and prepares a locking circuit to relay C, Fig. 4. Upon the operation of relay E over the circuit described, at armature e1 a point is opened in the circuit of the marking relay IFM so as to prevent its operation'until the end of the impulsing period, at armature 'c2 earth is connected to conductor 11 which extends to the third telephone conductor, at armature e3 an alternative operating circuit is provided to the impulsing relay I, at armature e4 a further point is prepared in the impulsing circuit to the counting relays, and at armature e5 a point is opened in an articial load circuit which is normally connected to the battery eliminator equipment shown in Fig. 5 the function of which will be fully described later. No further circuit operations will now take place until the calling subscriber allows his dial to return to normal under the influence of the restoring spring. When this is done impulses corresponding to the displacement of the finger plate of the dial will be delivered over the circuit previously traced to effect the operation of the impulsing relay I, Fig. 4. Upon the first de-energization of the impulsing relay I the following circuit may be traced to the counting relays: earth at the resting armature il, operated armatures b2 and e4, resting armatures of all counting relays, upper Winding of relay P to battery and in parallel by way of the resting armature c2 and winding of relay C to battery. Relays C and P will therefore operate their light armatures in this circuit, the former relay completing a locking circuit to earth at the operated armature b4, whilst the latter relay prepares a circuit for its full operation on both windings in series at the cessation of the impulse. It should be mentioned that a straightforward numbering scheme is employed for the system namely 1 to 0 and assuming for example that in this particular instance the digit 7 is dialled in order to establish connection with the telephone bearing that number, the following counting operation will be performed on the counting relays P to U. At vthe cessation of .the vfirst impulse, relayP willbelfully operated lon'both windings in series to .tran-sier the impulsing circuit -to a relay Q, and at vthe cessation of `the Asecond impulse relay Q willbe fullyoperated'in a similar vinanner to transfer the -impulsing circuit to lrelay R, while Iat the same time opening the holding circuit Vto thepreceding relay IP which Vthen releases. This sequence of operations will be observed in respect o'f Aall relays P to U until the last relay U is operated at .the cessation of the y6th impulse. Relay U it will lbe vobserved.remains-locked during the subsequent yimpulsing since its holding circuit -does not .includea resting-armatureof the following relay. Upon the Areception of the .7th

and last impulse, relay P willagain be operated andlocked for an indenite period. It. should ybe mentioned that Vrelays B and E are equipped with copper slugs so as to enable them to hold operated continuously during the impulsing period. At the end of the impulsing period, relay I will holdand therefore after asuitable delay relay E will release lto complete the following 4circuit to the marking relay '.I'FM.: earth at the operated armaturelbB, operated armature c5, resting armature el, conductor 20, -operated armature Isc., winding of relay IFM, Fig. 3, to battery. Relay` IFM in operating. over the circuit described, at armature lfm'l operates .the upper -battery feed relay lFBin series with resistance YB, at armatures 'IfmZ to ljmll prepares a point in the -cir cuit of the right-'hand windings of rall `connecting relays LP, at armature lfmIZ prepares .analternati-Ve looking circuit for itself which will become operative in case a busy condition is encountered, at armature 1fml3 opens a point in the. locking circuit to Ithe connecting relay ILP and at armature Ifmli prepares :the circuit for ringing 4the called subscribers bell.

I'l'1.e.u`rning again to the counting relays in Fig. 4, it will be rememberedthatrelays 'P and U are operated, and therefore upon the release of relay E which is followed. by the operation of relay IFM, a circuit maybe traced to effect the operation ofthe connecting relay "LP associated with the telephone No. 7 from earth at the operated armature b3, operated armature c5, resting armature el, operated armature ul, resting armatures s4, r4 and Q4, operated armature p3, marking conductor v'7, resting armature "llq'l, resting armature 71327, operaed armature lfm, right-hand winding of relay rlLP to battery. The relays with .prefix '7 are not shown but ,are similar to the corresponding ones with prefix l2. lfm corresponds to 1'fm3. Since all. telephone circuits are identical the following description Yinstead of being vdirected to telephone No. '7 will bedirected to telephone No. 2 which is shown in the drawings. Upon the assumed operation of the connecting relay 2LP over the marking circuit traced Ait will be seen that telephone No. 2 is connected to ar calling circuit.

Considering now the operation of the battery feed relay IFB, 4at armatures Ifhl and .Ifb2 the battery feed circuit to the speaking conductors is completed, and at armature lfb a holding circuit is maintained toy the connecting relays LP. The lower battery feed relay IFA is also operaed in series with relay lFB'oVer the-speaking vcircuit and at armature Ifal a locking circuit Iis .prepared to relay IFM which is operative when the Wantedline is busy. The called subscribers bell will now vbe rung over the following circuit: ringingY current supply over lead 2.1 from the eliminator equipment shown in Fig. 5, winding of ringing cut-off relay 1F, operated armature Ifml, conductor I3, operated armature 2Zp5, third telephone conductor, called subscribers bell :2RNG, condenser, resting switchhook springs ZSWHI, right-hand speaking conductor, operated armature 2Zpl,conductor 14, operated armature Ifb2, Fig. 3, to earth return by wayof the Winding of the battery feed relay IFA. The calling subscribers bell is of course not responsive to the ringing circuit owing to the opening of the bell v circuit at the operated switchhook sp-rings ISWHI, but it should Ibe mentioned Vthat a distinctive tone indicative of the ringing condition is induced into the receiver of the calling instrument owing to the presence of alternating current in the common battery feed circuit formed by way of the winding of relay IFA. It should be` meniioned that the Winding of the ringing cut-off relay 1F is connected in parallel with a metal rectifier IMRC of a suitable Well-known type-so as te render the relay operable in the ringing cir-cuit. Relay IF in 'turn operates the vrelief relay IFR, which at armature lfrl provides an alternating holding circuit to the connecting relays LP during the interval between the release of relays ISC and IFM, and at armature lfr2 provides an alternative circuit Ito relay lIF. In lthis manner thev called subscribers bell is rung, and the foregoing conditions will obtain until thecall is answered. l

Returning again -to the-operation of relay IFB, it will be seen that at arma-.ure lfbl thecircuit is opened to the irnpulsing relay I, Fig. fi, which releasesand in turn drops .relay B after its slow period. Relay B in releasing drops the-counting relays which were operated and also relay C,

while at the same instant opening the `holding circuit to relays-IFM and ISC which also restore.,

The selector equipment, Fig. 4, -is therefore freed -for use-on other calls, andthe-control ofthe connections set up is transferred to the battery feed relay IFB. When now the called subscriber removes his lreceiver to answer the call the associated switchhook contacts ZSWHI will be operated lo open the ringing circuit and connect 'up the speaking circuit, and under those conditions relay 1F will release and in turn release lERvso as to disconnect the ringing current from the common conductor I3. Conversation may now take place between the calling and `called subscribers andthe necessary battery feed iorspeaking is supplied via the windings of thecomparativelyr high impedance relays IFA and IFB which also serve the purposeof controlling the connectio-n. AIn order to obviate the possibility-of inductive interference in the speaking circuit a special battery feeder is provided for the eliminator equipment, Fig. -5,. arranged, to feed the relays IFA, rIFB and ZFA, 233B, and this feeder is quite separae and distinct from that Iarranged -to supply the various controlling relays.

Considering now the possibility of the vwanted subscriber being already engaged on a'call, then the circuit operations lpreviously ydescribed will obtain so far as the release of relay E at the end of the impulsing period, whereupon the relays -lFll/I, IFB and IFA will be operated inthe manner explained. and since the ringingcirouit to the Wanted subscribers bell is lopened at the switchhook springs relays IF and IFR will -notbe operated so that under these conditions a holding circuit is established for relay IFM by way of the resting armature lfrl. the lrelease lof the impulsing relay I and lin 4turn relays B and C, the holding circuit is opened Following upon Y to relay 1SC which alsoreleases, and owing to the continued operation of relay lFM the holding circuit for the calling subscribers connect-ing rclay 1LP will not materialize via resting armature lfml3 and the relay is therefore released, The release of the connecting relay lLP is followed by the release of the feed relays lFA and lFB and also the marking relay lFM, and it will therefore be realized that the apparatus is now freed for use on further calls. Absence of ringing tone in the calling subscribers receiver is indicative that the call has not matured, and therefore further manipulation of the dial will be necessary to register a further attempt. Although the selector equipment, Fig. 4, is only engaged for a very short period of time during each call, approximately equivalent to the time taken for the operation of the dial and its return to normal, it is conceivable, however, that the second caller may remove his receiver and .operate the dial during this period, and therefore means must be provided to prevent the mutilation of the first callers connection. This possibility is guarded against by the provision of a special relay TE which is only operable during the period when the selector equipment is already engaged. Considering now the detailed circuit operation, it will be remembered from the previous description that upon seizure of the selector equipment, relay B is operated and therefore the shortcircuiting path is disconnected from relay TE. Consequently in case a second caller should remove his receiver and operate the dial, the connecting relay LQ associated with the second circuit will operate, but since relay 2SC is adjusted not to operate in series with relay TE, the impulsing relay I is not connected to the second circuit and therefore the first callers impulses will continue without interruption. It will be seen from the circuit that the second callers impulses are without effect and consequently since relay TE is held operated under the control of the offnormal springs of the dial so as to abs-orb all the impulses, there is no possibility of the second caller establishing a mutilated connection in case the selector equipment should become free meanwhile.

Referring now particularly to the battery eliminator equipment shown in Fig. 5, it will be l seen that this is of conventional form comprising a composite transformer XFR and a rectify-y ing unit RCU arranged to provide full wave rectication. The conductors 22 and 23 connecting with the left-hand winding of the transformer XFR extend to the alternating current mains supply, while the right-hand winding is split and arranged so that the upper'portion is stepped down to a pressure of conveniently volts for supplying ringing current to the equipment at mains frequency by Way of the fuse RFS and conductor 21. Conductor 21 extends to the windings of the ringing relays 1F and 2F of both connecting circuits. The lower winding of the transformer is stepped down to provide a pressure of 1 conveniently 16 to 18 Volts, and arranged to supply alternating current to the rectifying unit RCU Where full Wave rectification will take place in order that a continuous current may be delivered by way of suitable chokes 10H and ZCI-I and conductors 24 and 25 to the control relays and battery feed relays respectively. Suitable values for the chokes may be for instance .3 of an ohm for lCH and l ohm for 2CH, while large capacity electrolytic condensers of conveniently ,2000 microfarads are also employed for smoothing the rectified current and are connected across the respective feeders in the positions indicatedby 1EC and ZEC. Since, however, such condensers can only be guaranteed to function correctly over long periods where a substantially constant voltage is maintained, it will be 4appreciated that their presence in this particular circuit is not altogether desirable unless some special means of maintaining constant voltage is provided, since it will be remembered from the previous description that the rectifying unit on an open circuit will produce a pressure of from 16 to 18 volts, while under load it is quite conceivable' that this pressure will drop as low as 12 Volts. It will be seen therefore that the pressure is likely to vary between quite wide limits and therefore'it is arranged that an artificial load shall be connected across the rectifying unitvunder normal conditions which is approximately equivalent to the load when the apparatus is in use. This load is provided in the form of two resistances YF and YG split in order to dissipate the head more readily and connected up in a circuit controlled by resting armatures of relays E and C, which it will be remembered are only operated when the low resistance counting relays which are responsible for the majority `of the load are in use. In this manner therefore, a substantially constant voltage is maintained across the electrolytic condensers lEC and ZECy so that their operating characteristics may be retained unimpaired for a long period.

I claim:

1. In a telephone system, a plurality of lines each equipped with a dial, impulse responsive 'i` equipment for completing connections between said lines, said equipment being normally disconnected from all of said lines, and means respon-- sive to the movement of one of said dials from its normal position for associating said equip- "i ment with the associated line. 2. In a telephone system, a plurality of ysubscribers lines each equipped with a dial, impulse responsive apparatus common to said lines and normally disconnected therefrom, and means for connecting said apparatus to a calling line responsive only to the operation of the dial associated with the calling line.

3. In a telephone system, a plurality of subscribers lines each equipped with an impulse sender, impulse responsive apparatus common to said lines and normally disconnected therefrom, and means for associating said apparatus with a calling line only during the time the impulse sender associated with the calling line isy operated.

4. In a telephone system, a plurality of sub-l scribers lines each consisting of two talking conductors and a third conductor,impulseresponsive apparatus common to said lines and normally =f disconnected therefrom, and means controlled over the third conductor of a calling line for connecting such line to said common apparatus.

5. In a telephone system, a plurality of subscribers lines each equipped with a dial, impulse j.

responsive apparatus common to said lines and normally disconnected therefrom, means for connecting said common apparatus to a calling line, and a circuit over which said means is controlled by the dial of the calling line. K

6. In a telephone system, a plurality of subscribers lines, impulse responsive apparatus,

common to said lines and normally disconnected therefrom, a plurality of relays for associating said common apparatus with calling lines, and

means responsive to the operation of one relay to connect the common apparatus with a calling line for rendering the remainder of said relays inoperative.

'7. In a telephone system, a plurality of subscribers lines, impulse responsive apparatus common to said lines and normally disconnected therefrom, two relays for associating said common apparatus with calling lines, a third relay, means responsive to the operation of one of said two relays for connecting said apparatus to a calling line and for preparing a circuit for said third relay, means responsive to an attempt by a second calling line to connect with said common apparatus for completing the circuit to said third relay, and means controlled by said third relay for preventing the operation of the other of said two relays.

8. In a telephone system, a plurality of substation telephone lines each consisting of two speaking conductors and an auxiliary conductor, automatic switching apparatus controlled over a calling line for establishing a connection to a called line, and means for transmitting ringing current over the auxiliary conductor of the called substation telephone line for signalling purposes.

9. In a telephone system, a plurality of subscribers lines, automatic switching apparatus for completing telephone connections between said lines, means for transmitting ringing current to a called line, a relay included in the ringing circuit and operated by the ringing current, and a contact on said relay for opening the ringing circuit When said relay releases due to an interruption of the ringing current.

10. In a telephone system, a plurality of subscribers lines, automatic switching apparatus for completing vtelephone connections between said lines, a ringing circuit over which ringing current is supplied to a called line including the winding of a relay, a rectifier connected in multiple with said relay so that the relay will be operated by ringing current, and a make contact on said relay included in the ringing circuit to open the circuit when said relay releases responsive to cessation of ringing current.

1l. In a telephone system, a plurality of subscribers lines, impulse responsive apparatus common to said lines and normally disconnected therefrom, means responsive to the initiation of a call over one of said lines for associating said commmon apparatus with the calling line, means in the common apparatus responsive to digit impulses transmitted from the calling line for establishing a connection, independent of said common apparatus, between the calling and called lines, and means for releasing said common apparatus in case the called line is busy.

12. In a telephone-system, a plurality of subscribers lines, two common talking circuits over which telephone connections between said lines are established, impulse responsive apparatus common to said talking circuits and normally disconnected therefrom, means responsive to the initiation of a call over one of said lines for connecting the calling line to one of said talking circuits and for connecting the common apparatus to the seized talking circuit, and means for disconnecting said common apparatus from said talking circuit when the called line has been connected thereto and in case the called line is busy.

13. In a telephone system, subscribers lines, two talking circuits over which connections are established between said lines, impulse responsive apparatus common to said talking circuits and normally disconnected therefrom, means responsive to the initiation of a call over one of said lines for connecting the calling line to one of said talking circuits and for connecting said common apparatus to the seized talking circuit, means responsive to the initiation of a second call for connecting the second talking circuit to said common apparatus, and means for rendering said last means inoperative until said common apparatus has been disconnected from said first talking circuit.

14. The combination with a telephone system including automatic switching apparatus for establishing f talking connections between subscribers lines of current supply means comprising a source of alternating current and a rectifying system, a conductor over which alternating current for ringing purposes is supplied to said apparatus, a second conductor over which direct current suitable for operating current is supplied, and a third conductor over which direct current suitable for talking current is supplied.

15. The combination with a telephone system including automatic switching apparatus for establishing talking connections between subscribers lines of a source of commercial alternating current and means for converting said alternating current into three kinds of current for use in connection with said automatic switching apparatus, namely, alternating current suitable for ringing purposes, direct current suitable for operating said apparatus, and direct current suitable for talking current.

16. In combination, a telephone system including automatic switching apparatus operable to establish talking connections between subscribers lines, a source of alternating current, a rectifying system for converting said alternating current into direct current suitable for use in operating said automatic switching apparatus, an artificial load normally connected to said rectifying system, and means for disconnecting said load during the operation of said automatic switching apparatus.

ERNEST COURTMAN OLIVER. 

